1. Field of the Invention
The present invention relates to an optical film for a display device and a display device having the same, and more particularly, to an optical film for reducing color shift, which has engraved or embossed lens parts in order to reduce color shift depending on the watching angle, and a display device having the same.
2. Description of Related Art
In response to the emergence of the advanced information society, components and devices related to photoelectronics have been significantly improved and rapidly disseminated. Among them, image display devices have been widely distributed as TVs, Personal Computer (PC) monitors, and the like. Moreover, attempts are underway to simultaneously increase the size and reduce the thickness of such display devices.
A Liquid Crystal Display (LCD) is one type of flat panel display, and displays images using liquid crystals. The LCD is widely used throughout the industry since it has the advantages of light weight, low driving voltage, and low power consumption compared to other display devices.
FIG. 1 is a conceptual view schematically showing the basic structure and operating principle of an LCD 100.
With reference by way of example to a conventional Vertical Alignment (VA) LCD, two polarizer films 110 and 120 are arranged such that their optical axes are oriented perpendicular to each other. Liquid crystal molecules 150 having birefringence characteristics are interposed and arranged between two transparent substrates 130, which are coated with transparent electrodes 140. When an electric field is applied from a power supply unit 180, the liquid crystal molecules move and are aligned perpendicular to the electric field.
Light emitted from a backlight unit is linearly polarized after passing through the first polarizer film 120. As shown in the left of FIG. 1, the liquid crystals remain perpendicular to the substrates when no power is applied. As a result, the light, which is in a linearly polarized state, is blocked by the second polarizer film 110, the optical axis of which is perpendicular to that of the first polarizer film 120.
In the meantime, as shown in the right of FIG. 1, when power is on, the electric field causes the liquid crystal to shift to a horizontal alignment parallel to the substrates, between the two orthogonally oriented polarizer films 110 and 120. Thus, the linearly polarized light from the first polarizer film is converted into another linearly polarized light of which the polarization is rotated by 90°, circularly polarized light, or elliptically polarized light while passing through the liquid crystal molecules before it reaches the second polarizer film. The converted light is then able to pass through the second polarizer film. It is possible to gradually change the orientation of the liquid crystal from the vertical orientation to the horizontal orientation by adjusting the intensity of the electric field, thereby allowing control of the intensity of light emission.
FIG. 2 is a conceptual view showing the orientation and optical transmittance of liquid crystals depending on the watching angle.
When liquid crystal molecules are aligned in a predetermined direction in a pixel 220, the orientation of the liquid crystal molecules looks different depending on the watching angle.
When viewed from the front left (210), the liquid crystal molecules look as if they are substantially aligned along the horizontal orientation 212, and thus the screen looks relatively brighter. When viewed from the front along the line 230, the liquid crystal molecules are seen to be aligned along the orientation 232, which is the same as the actual orientation of the liquid crystal molecules inside the pixel 220. In addition, when viewed from the front right (250), the liquid crystal molecules look as if they are substantially aligned along the vertical orientation 252, and thus the screen looks somewhat darker.
Accordingly, the viewing angle of the LCD is greatly limited compared to other displays that intrinsically emit light, since the intensity and color of light of the LCD varies depending on the watching angle. With the aim of increasing the viewing angle, a large amount of research has been carried out.
FIG. 3 is a conceptual view showing a conventional attempt to reduce variation in the contrast ratio and color shift depending on the watching angle.
Referring to FIG. 3, a pixel is divided into two pixel parts, that is, first and second pixel parts 320 and 340, in which the orientations of liquid crystals in the two pixel parts are symmetrical to each other. Both the liquid crystals oriented as shown in the first pixel part 320 and the liquid crystals oriented as shown in the second pixel part 340 can be seen. The intensity of light reaching the viewer is the total intensity of light from the two pixel parts.
When viewed from the front left (310), liquid crystal molecules in the first pixel part 320 look as if they are aligned along the horizontal orientation 312, and liquid crystal molecules in the second pixel part 320 look as if they are aligned along the vertical orientation 314. Thus, the first pixel part 320 makes the screen look bright. Likewise, when viewed from the front right (350), the liquid crystal molecules in the first pixel part 320 look as if they are aligned along the vertical orientation 352, and the liquid crystal molecules in the second pixel part 340 look as if they are aligned along the horizontal orientation 354. Then, the second pixel part 340 can make the screen look bright. In addition, when viewed from the front, the liquid crystal molecules are seen to be aligned along the orientations 332 and 334, which are the same as the actual orientations of the liquid crystal molecules inside the pixel parts 320 and 340. Accordingly, the brightness of the screen observed by the viewer remains uniform and is symmetrical about the vertical center line of the screen, even when the watching angle changes. This, as a result, makes it possible to reduce variation in the contrast ratio and color shift depending on the watching angle.
FIG. 4 is a conceptual view showing another conventional approach for reducing variation in the contrast ratio and color shift depending on the watching angle.
Referring to FIG. 4, an optical film 420 having birefringence characteristics is added. The birefringence characteristics of the optical film 420 are the same as those of liquid crystal molecules inside a pixel 440 of an LCD panel, and the orientation thereof are symmetrical with the orientation of the liquid crystal molecules. Because of the orientation of the liquid crystal molecules inside the pixel 440 and the birefringence characteristics of the optical film, the intensity of light reaching the viewer is the total intensity of light passing through the pixel 440 and the optical film 420.
Specifically, when viewed from the front left (410), the liquid crystal molecules inside the pixel 440 look as if they are aligned along the horizontal orientation 414, and the imaginary liquid crystals of the optical film 420 look as if they are aligned along the vertical orientation 412. The resultant intensity of light is the total intensity of light passing through the pixel 440 and the optical film 420. Likewise, when viewed from the front right (450), the liquid crystal molecules inside the pixel 440 look as if they are aligned along the vertical orientation 454 and the imaginary liquid crystals of the optical film 420 look as if they are aligned along the horizontal orientation 452. The resultant intensity of light is the total intensity of light passing through the pixel 440 and the optical film 420. In addition, when viewed from the front, the liquid crystal molecules are seen to be aligned along the orientations 434 and 432, which are the same as the actual orientation of the liquid crystal molecules inside the pixel 440 and the orientation of the optical film 420, respectively.
However, even if the approaches shown in FIGS. 3 and 4 are applied, there remains the problem as shown in FIG. 5. That is, a color shift still occurs depending on the watching angle, and the color changes as the watching angle increases.
In addition, the optical film and the display device, in particular, a TN mode LCD device of the related art, have problems of gamma-curve distortion and grayscale inversion.
The information disclosed in this Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.